PLC safety circuitry

Safety circuits are emergency circuit that must be provided to stop either partially or totally the operation of the controller or the controlled machine or process.These circuits should be hardwired outside the controller so that in the event of a total controller failure, The independent and rapid shutdown is available.

The safety requirements for installation:

The main disconnect switch is installed on the incoming power lines as a means of removing power from the entire programmable controller system. The main power disconnect switch should be located where operators and maintenance personnel have quick and easy access to it. Ideally, the disconnect switch is mounted on the outside of the PLC enclosure so that it can be accessed without opening the enclosure.

An isolation transformer is used to isolate the controller from the main power distribution system and step the voltage down to 120 VAC.

A hardwired master control relay is included to provide a convenient means for emergency controller shutdown. Because the master control relay allows the placement of several emergency-stop switches in different locations, its installation is important from a safety standpoint.

Over travel limit switches or mushroom head emergency stop pushbuttons are wired in series so that when one of them opens, the master control is de-energized. This removes power to input and output device circuits. Power continues to be supplied to the controller power supply so that any diagnostic indicators on the processor module can still be observed.